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Cascading Style Sheets, most of the time abbreviated in CSS, is a stylesheet language used to describe the presentation of a document written in HTML or XML (including various XML languages like SVG or XHTML). CSS describes how the structured element must be rendered on screen, on paper, in speech, or on other media.

CSS is one of the core languages of the open web and has a standardized W3C specification. Developed in levels, CSS1 is now obsolete, CSS2.1 a recommendation and CSS3, now split into smaller modules, is progressing on the standard track. The first early drafts of CSS4 modules are being written and reviewed.

Related Topics

Mozilla technologies which make extensive use of CSS: XUL, Firefox and Thunderbird extensions and themes.

News

Pointer Events reached the Candidate Recommandation status, meaning that the CSS property touch-action, currently only implemented in IE10 (with the -ms- prefix), is no more experimental. (May 6th, 2013)

Gecko's support of flexible boxes has been adapted to match a recent specification clarification: from Firefox 23, and already in Nightly, {{cssxref("::before")}} and {{cssxref("::after")}} will be flex items, and as such can be repositioned using {{cssxref("order")}} and {{cssxref("align-self")}}. (May 3rd, 2013)

The CSSWG published a new working draft of the Selectors Level 4 specification. It still is in the exploring phase and most of the new features are not supported anywhere, but it refines the behavior of scoped selectors and added two new ones: :blank, a relaxed {{cssxref(":empty")}} matching elements only containing spaces and carriage returns, and :placeholder-shown. The two pseudo-classes :matches() and {{cssxref(":not", ":not()")}} have been tweaked to allow more complex parameters. But beware, as this is still a very early draft, these features may change in the future. (May 2nd, 2013)

Getting help from the community

You need help on a CSS-related problem and can't find the solution in the documentation?

Revision Source

<p><span class="seoSummary"><strong>Cascading Style Sheets</strong>, most of the time abbreviated in <strong>CSS</strong>, is a <a href="/en-US/docs/DOM/stylesheet">stylesheet</a> language used to describe the presentation of a document written in <a href="/en-US/docs/HTML" title="The HyperText Mark-up Language">HTML</a></span> or <a href="/en-US/docs/XML" title="en-US/docs/XML">XML</a> (including various XML languages like <a href="/en-US/docs/SVG" title="en-US/docs/SVG">SVG</a> or <a href="/en-US/docs/XHTML" title="en-US/docs/XHTML">XHTML</a>)<span class="seoSummary">. CSS describes how the structured element must be rendered on screen, on paper, in speech, or on other media.</span></p>
<p>CSS is one of the core languages of the <em>open web</em> and has a standardized <a class="external" href="http://w3.org/Style/CSS/#specs">W3C specification</a>. Developed in levels, CSS1 is now obsolete, CSS2.1 a recommendation and <a href="/en-US/docs/CSS/CSS3" title="CSS3">CSS3</a>, now split into smaller modules, is progressing on the standard track. The first early drafts of CSS4 modules are being written and reviewed.</p>
<div style="margin:auto;text-align:center;">
<div class="callout-box action-driven">
<div>
CSS Reference</div>
<p>An <a href="/en-US/docs/CSS/CSS_Reference" title="en-US/docs/CSS/CSS_Reference">exhaustive reference</a> for <u>seasoned Web developers</u> describing every property and concept of CSS.</p>
</div>
<div class="callout-box action-driven">
<div>
CSS Tutorial</div>
<p>A <a href="/en-US/docs/CSS/Getting_Started" title="en-US/docs/CSS/Getting_Started">step-by-step introduction</a> to help <u>complete beginners</u> get started. It presents all the needed fundamentals.</p>
</div>
<div class="callout-box action-driven">
<div>
CSS3 Demos</div>
<p>A <a href="/en-US/demos/tag/tech:css3" title="https://developer.mozilla.org/en-US/demos/tag/tech:css3">collection of demos</a> showing the <u>latest CSS technologies</u> in action: a boost for the creativity.</p>
</div>
</div>
<div class="row topicpage-table">
<div class="section">
<h2 class="Documentation" id="Documentation" name="Documentation">Documentation and tutorials about CSS</h2>
<dl>
<dt>
CSS key concepts</dt>
<dd>
Describes the <a href="/en-US/docs/CSS/Syntax" title="/en-US/docs/CSS/Syntax">syntax and forms of the language</a> and introduces fundamentals like <a href="/en-US/docs/CSS/Specificity" title="Specificity">specificity</a> and <a href="/en-US/docs/CSS/inheritance" title="inheritance">inheritance</a>, the <a href="/en-US/docs/CSS/box_model" title="Box model">box model</a> and <a href="/en-US/docs/CSS/margin_collapsing" title="Margin collapsing">margin collapsing</a>, <a href="/en-US/docs/CSS/Understanding_z-index/The_stacking_context" title="The stacking context">stacking</a> and <a href="/en-US/docs/CSS/block_formatting_context" title="block formatting context">block-formatting</a> contexts, or the <a href="/en-US/docs/CSS/initial_value" title="initial value">initial</a>, <a href="/en-US/docs/CSS/computed_value" title="computed value">computed</a>, <a href="/en-US/docs/CSS/used_value" title="used value">used</a> and <a href="/en-US/docs/CSS/actual_value" title="actual value">actual</a> values. Entities like <a href="/en-US/docs/CSS/Shorthand_properties" title="CSS/Shorthand_properties">CSS shorthand properties</a> are also defined.</dd>
<dt>
<a href="/en-US/docs/Web/Guide/CSS" title="/en-US/docs/Web/Guide/CSS">CSS developer guide</a></dt>
<dd>
Articles to help you learn CSS techniques to make your content shine.</dd>
</dl>
<h2 class="Tools" id="Tools" name="Tools">Tools easing CSS development</h2>
<ul>
<li><span class="external">The </span><a class="external" href="http://jigsaw.w3.org/css-validator/">W3C CSS Validation Service</a> checks if a given CSS is valid. It is an invaluable debugging tool.</li>
<li>Firefox' <a class="link-https" href="https://addons.mozilla.org/en-US/firefox/addon/1843">Firebug extension</a><span class="external">, a popular</span> extension of that navigator that allows to edit live CSS on watched sites. Very practical to test some changes, though this extension does much more.</li>
<li><span class="external">Firefox' </span><a class="link-https" href="https://addons.mozilla.org/en-US/firefox/addon/60">Web Developer extension</a> also allows to watch and edit live CSS on watched sites. Simpler than Firebug, though less powerful.</li>
<li>Firefox' <a class="external link-https" href="https://addons.mozilla.org/en-US/firefox/addon/179">EditCSS extension</a> allows editing CSS in the sidebar.</li>
</ul>
<p>&nbsp;</p>
<h2 class="Related_Topics" id="Related_Topics" name="Related_Topics">Related Topics</h2>
<ul>
<li>Mozilla Learn <a href="/en-US/learn/css" title="https://developer.mozilla.org/en-US/learn/css">CSS resources</a>.</li>
<li>Open Web languages on which CSS is often applied: <a href="/en-US/docs/HTML" title="en-US/docs/HTML">HTML</a>, <a href="/en-US/docs/SVG" title="SVG">SVG</a>, <a href="/en-US/docs/XHTML" title="en-US/docs/XHTML">XHTML</a>, <a href="/en-US/docs/XML" title="en-US/docs/XML">XML</a>.</li>
<li>Mozilla technologies which make extensive use of CSS: <a href="/en-US/docs/XUL" title="en-US/docs/XUL">XUL</a>, Firefox and Thunderbird <a href="/en-US/docs/Extensions" title="en-US/docs/Extensions">extensions</a> and <a href="/en-US/docs/Themes" title="en-US/docs/Themes">themes</a>.</li>
</ul>
</div>
<div class="section">
<h2 class="Related_Topics" id="News" name="News">News</h2>
<ul>
<li><a href="http://www.w3.org/TR/pointerevents/" title="http://www.w3.org/TR/pointerevents/">Pointer Events</a> reached the <em>Candidate Recommandation</em> status, meaning that the CSS property <code>touch-action</code>, currently only implemented in IE10 (with the <code>-ms-</code> prefix), is no more experimental. <em>(May 6th, 2013)</em></li>
<li>Gecko's support of <a href="/en-US/docs/CSS/Tutorials/Using_CSS_flexible_boxes" title="/en-US/docs/CSS/Tutorials/Using_CSS_flexible_boxes"><em>flexible boxes</em></a> has been adapted to match a recent specification clarification: from Firefox 23, and already in Nightly, {{cssxref("::before")}} and {{cssxref("::after")}} will be flex items, and as such can be repositioned using {{cssxref("order")}} and {{cssxref("align-self")}}. <em>(May 3rd, 2013)</em></li>
<li>The CSSWG published a <a href="http://www.w3.org/TR/2013/WD-selectors4-20130502/">new working draft</a> of the Selectors Level 4 specification. It still is in the <em>exploring phase</em> and most of the new features are not supported anywhere, but it refines the behavior of scoped selectors and added two new ones: <code>:blank</code>, a relaxed {{cssxref(":empty")}} matching elements only containing spaces and carriage returns, and <code>:placeholder-shown</code>. The two pseudo-classes <code>:matches()</code> and {{cssxref(":not", ":not()")}} have been tweaked to allow more complex parameters. But beware, as this is still a very early draft, these features may change in the future. <em>(May 2nd, 2013)</em></li>
</ul>
<h2 class="Community" id="Community" name="Community">Getting help from the community</h2>
<p>You need help on a CSS-related problem and can't find the solution in the documentation?</p>
<ul>
<li>Check the <a href="/en-US/docs/CSS/Common_CSS_Questions" title="en-US/docs/CSS/Common_CSS_Questions">common CSS questions</a> that give hints to solve common CSS problems.</li>
<li>Go to <a href="http://stackoverflow.com/questions/tagged/css" title="http://stackoverflow.com/questions/tagged/css">Stack Overflow</a>, a collaboratively built and maintained Q&amp;A site and look if you can find the answer to your question. If not you will be able to ask your question there.</li>
<li>Consult the layout forum, which covers CSS and HTML: {{ DiscussionList("dev-tech-css", "mozilla.dev.tech.layout") }}
<ul>
<li>Ask your question on the Mozilla IRC channel: <a class="link-irc" href="irc://irc.mozilla.org/css">#css</a></li>
<li>Ask your question on the <a class="external" href="http://www.css-discuss.org/">CSS-Discuss site and list</a></li>
</ul>
</li>
</ul>
</div>
</div>
<p>&nbsp;</p>